1. Field of the Invention
The present invention relates to communications networks. More specifically, the present invention relates to an apparatus and method for controlling restorative switching of an optical network.
2. Related Art
A service disruption in a communications network may be caused by a number of problems including inoperable communications links, cable cuts, or failure of equipment within a system. In the event of a failure the traffic must be restored temporarily until the failure is repaired. Two restoration approaches are network restoration switching and line protect switching. The restoration approach which is used varies depending on the failure.
Network Restoration Switching: When a fiber cut or other major failure disables a communications system, network restorative switching (NRS) may be employed to reroute the traffic through the network via a restoration path. The restoration path carries traffic between the two end nodes until repairs of a path normally supporting the system are complete.
A mesh topology is a preferred topology for a network using NRS. A properly equipped mesh network contains switching nodes that are connected to two or more adjacent nodes. In a mesh topology, a given signal may have many possible routes by which to traverse the network. Mesh networks allow for sophisticated actions to be taken in response to a failure. If there is a sudden failure of several links, or an entire span, neighboring switching nodes can perform distributed switching to divert the traffic around the failure. For this purpose, most of the spans in a mesh network are equipped with extra spare links that can be called upon for emergency traffic handling in response to a failure.
The manner in which a given network should switch to recover from any possible failure is presently the subject of considerable development in the communications field. One type of network restoration scheme locates a service disruption, identifies alternate routes, and then establishes such routes, in order that a service disruption will minimally affect a communication system user. Two types of control schemes for mesh restoration are a centralized scheme and a distributed pre-planned scheme.
In a centralized restoration scheme, a telecommunication network includes a central site capable of establishing alternate routes when a failure occurs. For an example of a centralized restoration scheme see U.S. Pat. No. 5,182,744 issued to J. Askew el al., incorporated in its entirely herein by reference.
In a distributed pre-planned restoration scheme, network connections are restored by intelligent switching nodes distributed throughout the network. For an example of a distributed pre-planned restoration scheme see U.S. Pat. No. 5,173,689 issued to T. Kusano, incorporated in its entirety herein by reference.
When a disruption occurs, the time to restore service using NRS depends upon a number of factors, such as (a) the time required to identify the locations of the service disruption; (b) the time required to determine alternative routes that avoid the service disruption; and (c) the time required to actually establish such routes. In selecting a new communications route, it is often desirable to select the most efficient alternate route, i.e., the one generally having the minimal distance and/or hop count.
Line Protect Switching: Another method of network restoration is line protect switching (LPS). In LPS, local light termination elements (LTE) reroute traffic from a "working" channel to a "spare" channel. LPS ensures resiliency to equipment failure by employing a spare channel, also referred to as the protect channel, that normally does not carry traffic but may be used as a back-up should a "working" channel fail. The spare channel can be rerouted along the same physical path as the working channel. Preferably, the spare channel is routed along a completely different physical path to minimize the chance that the spare channel experiences the same fate that disabled the working channel.
In order to reduce costs, LPS can employ the use of one spare channel for five working channels. Because fewer spare channels are available than working channels, LPS cannot restore an extensive failure, such as a cable cut. LPS is primarily aimed at restoring single channel failures and is implemented within the LTE, which is the local equipment that terminates the fiber optic cable. Since LPS is localized and simple, it is also very fast requiring only tens of milliseconds for restoring a failed communications system. LPS can quickly restore simple localized failures. In many applications, the LPS can switch traffic without causing any significant interruption to traffic. LPS is described in further detail in commonly-owned, co-pending U.S. application Ser. No. 08/672,808 entitled, "System and Method for Photonic Facility and Line Protection Switching," filed by John Fee on Jun. 28, 1996, incorporated in its entirety by reference herein.
Because telecommunications networks can include high capacity terrestrial and under water optical cables, the networks are susceptible to failures that disable a very large number of channels causing potential high volume of traffic loss and significant economic impact. There is a need for a fast, efficient, and reliable apparatus and method for restoring a network. Additionally, there is a need for an apparatus and method for restoring a network that takes advantages of the benefits of both LPS and NRS.